DESCRIPTION: Since the early loss of mismatch repair has been shown to be an early mutation driving colon tumorigenesis, the applicant hypothesizes that the loss of nucleotide excision repair (NER) is associated with breast cancer etiology. Preliminary results show a significant loss of NER in early stage breast tumor and ductal carcinoma in situ (DCIS). In Aim 1 of this revised proposal a detailed analysis of NER on statistically significant numbers of tumor and non-tumor breast epithelium using the functional assay for NER, the unscheduled DNA synthesis (UDS) will be performed. The second aim will involve NER analyses on tumors ranging from pre-invasive DCIS to stage IV breast carcinomas. In Aim 3, the applicant will perform a second assay for NER, the host cell reactivation (HCR) assay which measures NER on a UV damaged transfected reporter gene construct on a subset of breast tumor and normal breast epithelial cells to verify and refine the data specifically on active gene repair. In the last aim, the applicant will analyze the underlying molecular mechanism of the reduced repair in stage I tumors by determining the expression levels of the cloned NER genes using quantitative ribonuclease protection assays and in situ hybridization.